Rapid curing melamine-formaldehyde impregnated paper sheet for producing surface of improved stain resistance and luster retention

ABSTRACT

An aqueous solution of a slightly hydrophobic, unmodified reaction product of formaldehyde and melamine in a molar ratio from 1:1 to 1.9:1 and, as a curing catalyst, a water-soluble acidic salt of aluminum or zinc, which solution remains ungelled for at least 12 hours at ordinary temperatures, is used to impregnate a cellulosic paper sheet. The dried sheet forms a surface lamination cures in 1 to 2 minutes at 315* F. to a surface having superior stain-resistance and luster retention.

United States Patent Meiser [4 1 Mar. 21, 1972 [54] RAPID CURINGMELAMINE- FORMALDEHYDE IMPREGNATED PAPER SHEET FOR PRODUCING, SURFACE OFIMPROVED STAIN RESISTANCE AND LUSTER RETENTION [72] Inventor: Kenneth D.Meiser, Dallas, Tex.

[73] Assignee: Plastics Manufacturing Company, Dallas,

Tex.

[22] Filed: Aug. 10, 1970 [21] Appl. No.: 62,648

Related U.S. Application Data [63] Continuation-impart of Ser. No.745,130, July 16, 1968, abandoned, and a continuation-in-part of Ser.No. 473,815, July 21, 1965, abandoned.

s2 U.S.Cl ..l17/l55L,260/29.4,260/67.6, 161/263 511 1nt.Cl. ..D2lh1/40[58] Field of Search..... 1 12/155 L; 260/294, 67.6;

Primary Examiner-Murray Katz Assistant Examiner-M. R. LusignanAttorney-Marshall and Yeasting [57] ABSTRACT An aqueous solution of aslightly hydrophobic, unmodified reaction product of formaldehyde andmelamine in a molar ratio from 1:1 to 1.9:1 and, as a curing catalyst, awater-soluble acidic salt of aluminum or zinc, which solution remainsungelled for at least 12 hours at ordinary temperatures, is used toimpregnate a cellulosic paper sheet. The dried sheet forms a surfacelamination cures in l to 2 minutes at 315 F. to a surface havingsuperior stain-resistance and luster retention.

2 Claims, No Drawings This is a continuatiomjn-part of applicationsjer.No.

473,815, filed July 2l,jl'.965,'and s r u'o. 745,130, filedluly 16,1968, both of are now abandoned.

KGR N ORTHE INVENT This inventionrelates to a melamine-formaldehydeimpregnated paper sheet which; substantially stable. in storage,

and whichforrns a surface laminationthatcuresrapidlyto provide a surfacehaving 1 superior,stain resistance and; luster retention. Because ofthese properties, the impregnated paper sheet of the present. inventionis particularly advantageous for use as a surfacelamination in@eplflductiqh ofmolded dinnerware. I I 7' 1 .One of themost importantcommercial usesof melamineformaldehyde resins is in the production ofmolded dinnerware. Approximately 80 percent" of the dinnerware setsproduced for houjsehold use from melamine-formaldehyde resins containdecorated pieces which areproduced witha' surface lamination of paperimpregnated with a melaminerformaldehyde reaction product; Typicallythesheetof paper which is to form the surface lamination is printed onitsunderside with a design, which is usually inseveral colorsjn thesubsequent molding operation, amolding cornposition isused to,form thebodylo fthe plate or other article, and. .the impregnated papcrsheet isusedztoforrn a surfacelaminationon one side of the article. In themolded article this surface lamination is substantially transparent, sothat the printed design is very clearlyvisible andyet isprotectedbecauseit is on the undersideof thelaminatio'nl Decorateddinnerware madeby this known methodhas the disadvantagethat thedecoratedsurface formed by-thesurface lamination'hasless initial luster andismoresubject-to staining and loss of luster during use than a surface of themolded article on which no surface lamination :has been used. In otherwords, the use of a surface lamination on such amolded article has madeit possible to decorate the article with adesign, u o y by sas ifl 's thw w ssista s i t lu t r and stain-resistance, initial luster and lusterretention of the decorated surface has been particularly undesirablebecause the decorated surface, such as the upper surface ofa plate, .isthe surface which is usually exposedto view during the use of thearticle. These deficiencies have made such decorated dinnervvaregenerally unacceptable for commercial applications such as use inrestaurants or institutions. In such commercial applications stainingand loss of luster are serious problems, because each piece ofdinnerware is used more frequently than in household use. For thesereasons this type of decorated dinnerware, which is sold in largequantities for household use,-has found very little use in restaurants.

' In this known method of producing decorated dinnerware, the reactionproduct with which the paper surface lamination is impregnated is theproduct of the reaction of formaldehyde and melamine in a molar ratiobetween 2:1 and 2.5:]. A reaction product with this molar ratio is fastcuring, even in the absence ofa catalyst. In the known method, no curingcatalyst is incorporated in the fast curing reaction product with whichthe paper surface lamination is impregnated, and in the molding of anarticle of dinnerware the normal molding period of 1 to 2 minutes at 315F. is sufficient to complete the cure of the reaction product with whichthe paper surface lamination is impregnated.

One of the objects of the present invention is to improve the propertiesof the surface lamination of decorated dinnerware by using in thesurface lamination a product of the reaction of formaldehyde andmelamine in a molar ratio substantially less than the molar ratiobetween 2:1 and 2.5:! which has been it hasbeen found in general that assoon as an acidic substance I I luster retention of the decoratedsurface. Such sacrtfiee m ,the

used heretofore. However, the difficulty with a reaction product in alower molarformaldehyde-melamine ratio is that such a reaction productis much slower curing than a reaction product in. which the molar ratiois between 2:1 and 2.5:1. If

such a reaction product having a substantially lower molarformaldehyde-melamine ratio were used in place of the reaction productusually employed .in the surface lamination ofdecorateddinnerware,without. a curing catalyst, the cure of thisslowcuring reaction product would not be completed dur- .ing the moldingtime and at the molding temperature normally used in commercial practicefor the molding of decorated din- .ner,ware.

",The use of a slow curing reaction product of formaldehyde and-melaminein a relatively lowmolar ratio is described in :Example 7 of U.S.., Pat.No. 2,841,571. In that example the relatively slow, curing reactionproduct was curedat temperatures as, high as l7 5"v to 225 C. in ordertocomplete the cure in an, interval of 3 to 5 minutes. vWith those hightemperatures,

the sheets that were, impregnated with the slow curing reaction notcuredat suchhigh temperatures, because of thedanger of charting of thecellulose, .The temperature used for curing molded dinnerware, having apaper surface lamination, is about 315 R, which isequal to 157 C., andthe curing time at that temperature, is only 1 to 2 minutes.

It isknown thatan acidicsubstance will act as a catalyst to accelerate,the cure of a formaldehyde-melamine reaction product. In practice,however,,it has not been possible heretoforeto incorporate an acidicsubstance as a curing catalyst in an aqueous solution of aformaldehyde-melamine reaction product which is usedinan impregnatingapparatus, because ;is incorporated in an aqueous solution of aformaldehyde- -melamine reaction product, the reaction productimmediately begins to cure to an infusible state so that it quickly gelsor precipitates from the solution. An acidic substance which causesrapid precipitationorgelling cannot be incorporated in an aqueoussolution used in a commercial impregnating operation, because a solutionused in a commercial impregnating operation must remain stable for atleast 12 hours because the solution must be held in the tanks of theimpregnating apparatus during a working day, and at the end of theworking day the residue of solution in the tanks must remain stable longenough to permit a clean-up crew to drain and flush out the tanks.

Although it has not been possible in commercial practice heretofore toincorporate an acidic substance in an aqueous solution of aformaldehyde-melamine reaction product which is used in an impregnatingapparatus, it has been common practice to prepare a dry mixture of anacidic substance with a formaldehyde-melamine reaction product for useas a molding composition. In such a dry mixture, the reaction productand the acidic substance are present as discrete particles, so that theacidic substance has no appreciable catalytic action upon the reactionproduct until the molding composition is fused during the moldingoperation. The situation when an acidic substance is present in the formof discrete particles mixed with a dry reaction product is quitedifferent from the situation when the acidic substance has beenmolecularly dispersed in the reaction product by incorporating it in anaqueous solution. Thus U.S. Pat. No. 2,841,571 discloses theincorporation of a curing catalyst only in a dry molding composition,and does not disclose the incorporation of a curing catalyst in theimpregnating solution of Example 7.

US. Pat. No. 3,194,723 also states that melamine-femaldehyde resins havepoor stability in aqueous solutions in that the resins separatetherefrom upon standing for relatively short periods of time, e.g., 1-2days. (Even in the absence of an acidic curing catalyst).

SUMMARY OF THE INVENTION The principal object of the invention is theproduction of a dried paper sheet which forms a surface lamination thatcures in l to 2 minutes at 315 F. to a surface having superiorstainresistance and luster retention. Such a paper sheet is produced bydrying a cellulosic paper sheet which has been impregnated with anaqueous solution of a slightly hydrophobic, unmodified reaction productof formaldehyde and melamine in a molar ratio from 1:1 to 1.9:1containing, as a curing catalyst, a water-soluble acidic salt ofaluminum or zinc.

In accordance with the invention, it has been discovered that thesolution thus used in the practice of the invention remains ungelled forat least 12 hours at ordinary temperatures, and the dried paper sheetswhich have been impregnated with such a solution are substantiallystable so that they can be stored or shipped.

This discovery that the combination of a slightly hydrophobic,unmodified reaction product of formaldehyde and melamine in a molarratio from 1:1 to 1.921 with a curing catalyst which is a water-so1ub1eacidic salt of aluminum or zinc forms a stable aqueous solution is asurprising discovery, because the presence of an acidic substance otherthan a salt of aluminum or zinc would render such a reaction productunstable in an aqueous solution because it would cause the reactionproduct to gel or precipitate from the solution. The action of an acidicsalt of aluminum or zinc in combination with a slightly hydrophobic,unmodified reaction product of formaldehyde and melamine in a molarratio from 1:1 to 19:1 is very unusual, in that the aqueous solutionremains ungelled for at least 12 hours at ordinary temperatures, andpaper sheets which have been impregnated with aqueous solution and driedare substantially stable and yet form surface laminations that cure in 1to 2 minutes at 315 F. That is all the more surprising in view of thedisclosure of U.S. Pat. No. 3,084,071. That patent discloses that theincorporation of an acidic salt of aluminum or zinc in a water-solubleaminoplast resin with which a textile fabric is impregnated causes theaminoplast resin to cure in only 1 day, even after the impregnatedfabric has been dried, so that the fabric must then be rinsed in coldwater for minutes to remove the aluminum or zinc salt in order toprevent overcuring. The difference in results is accounted for by thefact that the aminoplast resin" used in accordance with the disclosureof U.S. Pat. No. 3,084,071 is a simple monomeric water-soluble reactionproduct, whereas the reaction product used in the practice of thepresent invention is a slightly hydrophobic reaction product offormaldehyde and melamine, which is a relatively complex condensationproduct of high molecular weight. The reaction products which are usedfor treating textile fabrics, as disclosed in U.S. Pat. No. 3,084,071,are simple crystalloidal substances. The use of similar crystalloidalreaction products for treating textile fabrics is disclosed in U.S. Pat.No. 2,839,429. These crystalloidal monomeric water-soluble reactionproducts are highly unstable in the presence of an aluminum or zinc saltin that they cure in only 1 day even in a dried textile fabric.

The combination of an acidic salt of aluminum or zinc with a slightlyhydrophobic, unmodified reaction product of formaldehyde and melamine ina molar ratio from 1:1 to 1.921, which is used in the practice of thepresent invention, is highly advantageous in that it forms an aqueoussolution that remains ungelled for at least 12 hours at ordinarytemperatures, and in that it remains substantially stable after papersheets have been impregnated with the aqueous solution and dried.

The stability of the dried paper sheets of the present invention isimportant, because impregnated paper sheets for use as surfacelaminations on molded dinnerware are articles of commerce which arecommonly produced in an impregnating plant, then shipped to a printerwho prints a suitable decoration upon the sheets, and then shipped to amolder who uses the printed impregnated sheets as surface laminations inthe molding of dinnerware. A formaldehyde-melamine reaction product withwhich such sheets are impregnated must remain stable during storage,handling and shipment of the sheets, so that when the impregnated papersheets are finally used in producing molded dinnerware, the reactionproducts with which the sheets are impregnated are still uncured andthus capable of fusing during the molding operation.

It has been found that a remarkable improvement in decorated dinnerwarecan be obtained by the use of a surface lamination consisting of theimpregnated paper sheet of the present invention. When the presentimpregnated paper sheet is used to form the surface lamination, thestain-resistance and luster retention of the resulting surface, insteadof being inferior to that of the unlaminated lower side of the moldedarticle, are actually superior to the stain-resistance and lusterretention of the lower side of the molded article.

Consequently, the present invention makes possible the production ofdecorated dinnerware which is satisfactory for use in restaurantsbecause it can be used indefinitely without appreciable staining andwithout appreciable loss of luster on the decorated or laminatedsurface.

An important requisite which is met by an impregnated paper sheetembodying the present invention is that it forms a surface laminationthat cures in 1 to 2 minutes at 315 F. The ability of the presentimpregnated paper sheet to cure in l to 2 minutes at 315 F. is essentialin order to made it possible to use the present impregnated paper sheetas the surface lamination on an article of molded dinnerware. That istrue because the body of an article of molded dinnerware, other than thesurface lamination, is formed from a standard formaldehyde-melaminemolding composition which must be formed by molding for 1 to 2 minutesat 315 F.

In other words, the conditions of l to 2 minutes at 315 F., under whicha surface lamination formed from a paper-sheet of the present inventionmust be cured, are fixed because those are the curing conditions whichmust be used for standard commercial formaldehyde-melamine moldingcomposition that forms the body of the article of molded dinnerware.Also, the molding temperature must be limited to 315 R, which is equalto 157 C., in order to avoid charring of the cellulose filler containedin the standard commercial molding composition or the cellulosecontained in the paper surface lamination. Thus it would not be possiblein the molding of an article of dinnerware to use the curing conditionsof 3 to 5 minutes at 175 to 225 C. which were employed in Example 7 ofU.S. Pat. No. 2,841,571 in order to complete the cure of aformaldehyde-melamine reaction product with which the heat-resistantfabric had been impregnated.

Thus the combination of an acidic salt of aluminum or zinc with aslightly hydrophobic, unmodified reaction product of formaldehyde andmelamine in a molar ratio from 1:1 to 1.9:] which is used in thepractice of the present invention has three advantages in that it formsan aqueous solution which remains ungelled for at least 12 hours atordinary temperatures, in that it remains substantially stable afterpaper sheets have been impregnated with the aqueous solution and dried,and in that the dried paper sheets form surface laminations that cure in1 to 2 minutes at 315 F. to a surface having superior stain-resistanceand luster retention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The first advantage achieved inthe practice of the present invention, which is the stability of theaqueous impregnating solution, has been demonstrated as follows.

EXAMPLE 1 A solution was prepared by adding 1 mole of melamine to anaqueous solution of 1.5 moles of formaldehyde and heating to about 200F. to obtain a clear solution having a pH between 8 and 9. The solutionwas then refluxed at atmospheric pressure until cloudiness appeared uponthe addition of 1 drop of the solution to a large volume of water at F.,indicating that a slightly hydrophobic reaction product had beenobtained. After being rapidly cooled to solids content of the solution,it is assumed that 1 mole of 5 water is split off during the reactionfor every 3 moles of formaldehyde.

The reaction product in this solution, having a molarformaldehyde-melamine ratio of 1.5:1 is slow curing. However,incorporation of an ordinary acidic curing catalyst in this solutionrenders it too unstable for commercial use.

In order to demonstrate the effect of incorporating an ordinary acidiccuring catalyst in the solution prepared as described above, lactic acidwas added to the solution in an amount equal to 0.6 percent by weight ofthe solution. The resulting composition had a pH of 6.7, and began togel in about 5 hours at ordinary temperatures after addition of thelactic acid, so that the solution containing the lactic acid could notbe used in a commercial laminating operation, in which a tank ofsolution must remain stable and ungelled throughout a working day whilesuccessive sheets or a continuous strip of paper is being impregnated.

Aluminum nitrate, instead of lactic acid, was added to another sample ofthe initial solution in an amount equal to 0.3 percent ofthe weight ofthe sample solution. The resulting composition had a pH of 7.15, and asheet of paper impregnated with the composition and dried, when used asa surface lamination, was found to cure in l to 2 minutes at 315 F., therate of cure being at least as rapid as the rate of cure of a sheetimpregnated with the unstable sample of solution containing lactic acid.The sample of solution containing the aluminum nitrate remained clear at75 F. for more than 7 hours. At this temperature, a slight hazedeveloped in about hours, and the solution became cloudy in about 18hours but was still usable in ordinary impregnating apparatus afterabout 20 hours.

EXAMPLE 2 Another composition was prepared from another part of the sameinitial solution by adding 0.6 percent by weight of zinc sulphate. Theresulting solution could be used to produce surface laminations in thesame manner as the solution containing aluminum nitrate, had a pH of7.4, and had substantially the same stability at 75 F. as the solutioncontaining aluminum nitrate.

PREPARATION OF AQUEOUS SOLUTION OF SLIGHTLY HYDROPHOBIC REACTION PRODUCTAn aqueous solution of a reaction product for use in the practice of theinvention may be prepared by dissolving 1 mole of melamine in an aqueoussolution containing from 1 to 1.9 moles of formaldehyde, theconcentration of the initial aqueous solution of formaldehyde being suchthat the reaction with the melamine produces a solution containing fromabout 45 to about 60 percent solids, assuming that 1 mole of water issplit off during the reaction for every 3 moles of formaldehyde. Beforethe melamine is added to the formaldehyde solution, a base such assodium hydroxide or an acid such as formic acid is added to theformaldehyde solution if necessary, in order that a solution having a pHfrom 8 to 9 may be obtained by addition of the melamine and heating toabout 200 F. The solution is then refluxed atatmospheric pressure untilcloudiness appears upon the addition of 1 drop of the solution to alarge volume of water at 100 F., indicating that a slightly hydrophobicreaction product has been obtained. Then the solution is cooled asrapidly as possible to room temperature.

As the molar ratio of formaldehyde to melamine used for the reactionapproaches 1.9, the benefits obtained in the practice of the inventiondecrease somewhat. On the other hand, as the molar ratio approaches 1:],it becomes more difiicult to obtain a clear solution. The preferredmolar ratio of formaldehyde to melamine for use in the practice of theinvention is from about 1.2:] to about 1.711. A solution prepared ashereinbefore described, using a molar ratio in this preferred range,will remain stable from 2 to 4 weeks if no curing catalyst has beenadded to the solution.

PRODUCTION OF IMPREGNATED PAPER SHEET An impregnating solution for usein the practice of the invention is prepared by adding to a solution ofa slightly hydrophobic reaction product, prepared as described above, alatent curing catalyst consisting of a water-soluble acidic salt ofaluminum or zinc. The amount of the latent curing catalyst added to thesolution varies with the particular catalyst employed, and may rangefrom 0.05 to 2.0 percent of the weight of the solution. Usually theamount of the latent curing catalyst added is from 0.1 to 1.0 percent ofthe weight of the solution.

The substance added to the solution as a latent curing catalyst may beany acidic salt of aluminum or zinc, such as aluminum acetate, aluminumlactate, zinc acetate, or any other water-soluble acidic salt ofaluminum or zinc with an organic acid. However, the preferred type oflatent curing catalyst for use in the practice of the invention is awater-soluble salt of aluminum or zinc with a strong mineral acid, suchas aluminum sulphate, aluminum chloride, aluminum nitrate, zincsulphate, zinc chloride or zinc nitrate.

The concentration of the impregnating solution used in the practice ofthe invention may be any concentration which makes it possible to impartthe desired amount of the melamine-formaldehyde reaction product to thepaper by an impregnating operation. Ordinarily the concentration of thesolution is from about 30 to about 70 percent solids.

The solution may be used to impregnate paper in the usual manner. Acontinuous web of paper may be fed from a supply roll, and may beimpregnated by means of coating rolls, or by passing it beneath thesurface of the solution. Preferably the paper is initially impregnatedby means of rolls and then is passed through the solution.

Because of the light color and excellent durability attainable by theuse of a melamine-formaldehyde reaction product, paper impregnated withsuch a reaction product is advantageously used as the surface sheet inthe production of the laminated article. Also the paper used forimpregnation with a melamine-formaldehyde reaction product usually is awhite paper, such as an alpha cellulose paper, in order to takeadvantage of the exceptionally light color of the melamine-formaldehydereaction product. The paper may consist of percent alpha cellulosefiber, or it may consist of a mixture of alpha cellulose fiber and rayonfiber, the proportion of each fiber usually being between 40 and 60percent.

The proportion of melamine-formaldehyde resin in the impregnated paper,measured on a dry basis, may range between 45 and 70 percent by weight,the remainder of the impregnated paper consisting of cellulose fibenThedried paper comprises 30 to 55 parts by weight of cellulosic fibersimpregnated with 45 to 70 parts by weight of the curable resin.

After the impregnating operation, the impregnated paper may be dried ata temperature from about 240 to about 280 F. for about 1 minute.

PRODUCTION OF LAMINATED ARTICLE A laminate produced by the use of suchimpregnated paper as the top sheet is useful for applications such asdoors, table topsand counter tops for which a permanent attractiveappearance is desired. The present invention makes it possible toproduce a surface of greatly improved stain resistance and lusterretention.

A particularly important application of the present invention is in theproduction of an impregnated paper sheet which is to form the ornamentalsurface on decorated dinnerware.

An impregnated paper sheet embodying the invention also may be used toform the surface layer in the production of a laminate in which the bodyof the laminate consists of a fibrous material such as particle boardcomposed of wood fiber.

EXAMPLE 3 Three solutions were prepared by reacting formaldehyde andmelamine in three different molar ratios, according to the followingprocedure.

Melamine (1,261.3 parts by weight) was dissolved in an aqueousformaldehyde solution which had been prepared as indicated in Table 1below. Before the addition of the melamine, sodium hydroxide had beenadded to the formaldehyde solution in an amount such that the pH of theclear solution obtained by adding the melamine and heating to 200 F. wasbetween 7.9 and 8.5. The reaction solution was then refluxed until theaddition of a drop of the solution to 100 cc. of water at 100 F. causedcloudiness to appear. The reaction solution was then cooled to roomtemperature. The total amount of water in each of the three reactionsolutions thus prepared was equal to the total amount of melamine andformaldehyde used for the reaction. Table 1 below shows the number ofparts by weight of 37 percent aqueous formaldehyde solution and thenumber of parts by weight of water used in preparing the initialformaldehyde solution in each case, and shows the number of moles offormaldehyde used for each mole of melamine.

An impregnating solution was prepared by addition ofa different latentcuring catalyst to each of several samples of each solution of amelamine-formaldehyde reaction product prepared in the manner describedabove. A different sheet of alpha cellulose paper was impregnated witheach of the resulting solutions in an amount such that the impregnatedpaper, after being dried, contained from 67 to 69 percent ofmelamine-formaldehyde resin. The excess of the solution remaining on thesurface of the impregnated paper was removed by means of scraper barsand pressure rolls, and the impregnated paper was dried at 270 F., forabout 1 minute. After the drying operation, samples of the impregnatedpaper were subjected to a standard test in which eight or nine sheets ofthe paper were pressed together at about 300 F. for about 2 minutes, andthe flash durrounding each laminated piece was then broken off andweighed. The weight of the flash was between 6 and 12 percent of thetotal weight of each laminated piece including the flash.

Each dried impregnated sheet was then used as a surface lamination inthe molding of a dinner plate. A commercial alpha cellulose filledmelamine-formaldehyde molding composition was used to form the body ofthe plate. A preheated preform of the molding composition was firstmolded at a temperature of 315 F. and under a pressure of about 3,000pounds per square inch for 45 seconds and the mold was then opened andthe impregnated paper sheet was placed on top of the molded piece. Themold was then closed to hold the piece under the same pressure andtemperature for the period of time specified in Table 2 below, and thefinished piece was ejected from the mold.

As a control, the same procedure was used to produce a molded plate witha surface lamination consisting of an alpha cellulose paper containingabout 67 percent by weight of a thermosetting product of the reaction offormaldehyde and melamine in a molar ratio between 2:1 and 2.521. Theimpregnated paper used for the control was an ordinary commercialimpregnated paper of a standard grade which is used to form a surfacelamination on molded dinnerware.

Each molded plate thus produced was sawed in half, and one-half of eachplate was tested by immersing it in a test bath which had been preparedby adding instant coffee to water in an amount equal to 96 grams pergallon of water. The test bath containing the samples was held at atemperature of 180 F. for 48 hours, and the half plates were thenremoved from the bath, rinsed with water and dried. The results of thetest were determined by measuring the degree of staining of thelaminated surface of each test piece. The degree of staining of eachtest piece was determined by comparing the reflectance for blue light(475 p.) of the laminated surface of the tested half of each plate withthe reflectance of the laminated surface of the untested half of theplate, using a Bausch and Lomb spectrophotometer (Spectronic No. 20).

Table 2 below, showing the results of a number of tests, gives thenumber of moles of formaldehyde used for each mole of melamine inpreparing the impregnating solution, the name of the catalyst, theamount of the catalyst in percent by weight of the reaction solution,the number of minutes for which the piece was cured at 315 F. afteraddition of the top sheet, and the stain-resistance expressed as thepercent reflectance for blue light of the laminated surface of the testpiece, taking the reflectance of the laminated surface of the untestedhalf of the plate as 100 percent.

TABLE 2 Control (Commercial product of standard grade) CH,O MinutesStain Moles Cure Resistance Between 2.0 2 52 and 2.5 1 46 AluminumChloride CH,O of Stain Resistance Moles Catalyst 1 min. cure 2 min. cureZinc Chloride CH,O 7c of Stain Resistance Moles Catalyst 1 min. cure 2min. cure Aluminum Nitrate CH,O 7c of Stain Resistance Moles Catalyst lmin. cure 2 min. cure Zinc Nitrate CH,O of Stain Resistance MolesCatalyst 1 min. cure 2 min. cure Aluminum Sulphate CH O of StainResistance Moles Catalyst 1 min. cure 2 min. cure Zinc Sulphate C11 k ofStain Resistance Moles Catalyst 1 min. cure 2 min. cure Zinc AcetateCH,0 52 of Stain Resistance Moles Catalyst 1 min. cure 2 min. cure Theresults given in Table 2 show that the staining of the laminated surfaceof the control pieces was relatively severe. Visual observation of thesecontrol pieces showed that the laminated surface of each test piece wasmore severely stained than the unlaminated underside of the piece, eventhough the staining of the underside was quite substantial.

The results given in the table also show that in the practice of thepresent invention, the use of a relatively low molarformaldehyde-melamine ratio such as 1311 gives superior stainresistance,but results in a product that is relatively slow curing. In commercialpractice, a cure time of 1 minute is quite satisfactory, but a cure timeof 2 minutes is on the borderline of acceptability.

Visual observation of the pieces having a laminated surface produced inaccordance with the present invention which were tested as describedabove showed that the staining and loss of luster on the laminatedsurface was barely appreciable, whereas the staining and loss of lusteron the underside of each piece was relatively quite substantial.

Other molded plates produced as described above, instead of beingsubjected to immersion in a coffee solution, were washed repeatedly in acommercial dishwasher, using various chlorinated caustic detergentswhich are commonly used in commercial dishwashing. Under theseconditions the laminated surfaces of the plates prepared in accordancewith the present invention retained their gloss and luster far longerthan the laminated surfaces of the control plates which had beenproduced by the use of a standard commercial impregnated paper. Loss ofgloss and luster is an indication that the resistantmelamine-formaldehyde resin surface has been leached away to expose thecellulose fibers. Once this has occurred the laminated surface issubject to very severe and rapid staining.

I claim:

1. A dried paper sheet which forms a surface lamination that cures in lto 2 minutes at 315 F. to a surface having superior stain-resistance andluster retention, which comprises 30 to 55 parts by weight of cellulosicfibers impregnated with a substantially stable composition consistingessentially of 45 to 70 parts by weight of a curable slightlyhydrophobic, unmodified reaction product of formaldehyde and melamine ina molar ratio from 1:1 to 1.9:1 and, as a latent curing catalyst, awater-soluble acidic salt of a metal of the class consisting of aluminumand zinc.

2. A dried paper sheet according to claim 1 wherein the molarformaldehyde-melamine ratio of the reaction product is from 1.2:1 to1.7:1, and the metal salt is a salt of a strong mineral acid.

2. A dried paper sheet according to claim 1 wherein the molarformaldehyde-melamine ratio of the reaction product is from 1.2:1 to1.7:1, and the metal salt is a salt of a strong mineral acid.